1. Field of the Invention
The present invention relates to a mold clamping mechanism for an injection molding machine and, more specifically, to improvements in a mold clamping apparatus for injection molding machine, provided with a means for intensifying mold clamping force.
2. Description of the Related Art
In a generally known mold clamping mechanism, a moveable platen is moved to clamp a mold between a stationary platen and the moveable platen. The mold clamping mechanism is capable of keeping the mold closed by a high clamping force against the pressure of a molten polymer injected into the mold. Straight hydraulic mold clamping mechanisms have prevalently been used for clamping a mold. Straight hydraulic mold clamping mechanisms include those of a boost ram system employing a hydraulic cylinder actuator having a cylinder, and a ram internally provided with a boost ram and axially slidably fitted in the cylinder, those of a side cylinder system employing a plurality of side cylinders in addition to a clamping cylinder and those of a pressure intensifying cylinder system employing a clamping hydraulic cylinder actuator and a pressure intensifying hydraulic cylinder actuator connected in series to the clamping cylinder. These known straight hydraulic mold clamping mechanisms need a clamping hydraulic cylinder actuator having a large capacity and a complicated hydraulic circuit to open and close a tank for storing the hydraulic fluid for operating the hydraulic cylinder actuator. The straight hydraulic mold clamping mechanisms of the boost ram system and the side cylinder system, in particular, need a large highpressure pump to exert a high clamping force on the mold, and the straight hydraulic mold clamping mechanisms of the pressure intensifying system has a relatively great longitudinal size.
A mold clamping mechanism disclosed in JPB2 No. 17851/1989 is provided with a screw mechanism and a cylinder actuator in combination. This mold clamping mechanism is provided with a first doubleacting cylinder actuator having a doubleacting ram, and firs and second pressure chambers, the volumes of which vary equally as the doubleacting ram moves, and a second doubleacting cylinder actuator having a pressure intensifying chamber. The first doubleacting cylinder actuator operates for opening and closing a mold with the first and the second pressure chamber communicating with each other. When clamping the mold, the first and the second pressure chamber are disconnected from each other, and the pressure in the pressure intensifying chamber is increased by the screw mechanism to drive the second doubleacting cylinder actuator. A hydraulic fluid is transferred from the first or the second pressure chamber to the second or the first pressure chamber by a pump to advance or retract the doubleacting ram when opening or closing the mold. Therefore, any additional tank for reserving the hydraulic fluid is not necessary.
A known electricdriven mold clamping mechanism is provided with a screw feed device which is driven by an ac servomotor to move a movable platen. In this electricdriven mold clamping mechanism, it is unavoidable that the ac servomotor must be of a large capacity and the screw feed device must be of a large size to exert a sufficiently high mold clamping force on the mold.
From this point of view, a prior art mold clamping mechanism disclosed in JPA No. 246806/1994 is provided with an improved servomotor as a driving source and an improved screw feed mechanism. This prior art mold clamping mechanism is provided with a pressure intensifying mechanism which pressures a hydraulic fluid contained in a sealed bag with a threaded rod having one end linked to an internally threaded nut rotated by a servomotor to produce a high mold clamping force. This pressure intensifying mechanism, however, is not reliable in surely holding the mold clamping force due to the leakage of the hydraulic fluid from the sealed bag.